Previous attempts to recycle used asphalt-aggregate compositions in conventional dryer drums have been generally unsuccessful. Recycling of used materials of this type are most desirable since the basic raw materials, asphalt and aggregate, are available in significant quantities in older roads and other "black top" surfaces that have settled, cracked and otherwise deteriorated because of long exposure to weather, heat extremes and weight loads. Gradual heating and mixing of the used materials and addition of certain compositions, especially make-up asphalt, in order to achieve proper or desirable asphalt-aggregate ratios and penetration characteristics, are required in the recycling process. Attempts to accomplish this in the conventional rotatable dryer drums in which hot flame is introduced are not successful because a portion of the particles high in asphalt content which are directly exposed to the flame and the extremely hot gases in the hottest portion of the drum are overheated thus becoming burned and coked. This not only undesirably degrades the asphalt, thereby substantially affecting the resulting product, but also causes smoke and other noxious fumes and volatiles to be driven directly into the atmosphere. The result is an inferior product and is undesirable from an air pollution standpoint. It is to the elimination of these problems that the present invention is directed.
In my aforesaid prior co-pending application, there is described an improved process and apparatus for treating asphalt and aggregate containing compositions, and especially used asphalt-aggregate composition to be recycled, comprising separating the composition into a plurality of different particle sizes ranging from coarse to fine, and introducing these individual different particle size portions into different zones of a conventional type dryer drum in which the composition is exposed to hot gases of combustion as it cascades along the rotating drum and is gravitationally directed therealong. In that invention there is particularly described a method whereby the coarse particles are introduced into the hottest end of the drum nearest the flame and hot gases of combustion whereas portions of smaller particle size ranges are introduced into one or more cooler zones within the drum away from the hottest gases of combustion and flame so as to avoid burning or degradation of the asphalt in the particles and which would result in an inferior product and the other problems set forth therein.